(a) Field of the Invention
The present invention relates to a multi-carrier code division multiple access (MC-CDMA) system. More specifically, the present invention relates to an MC-CDMA system for reducing a multiple access interference component in a frequency-selective fading channel environment.
(b) Description of the Related Art
In general, the MC-CDMA system uses orthogonal codes for multiple users or multiple data channels and carries orthogonal code chips multiplied by each data symbol on a different carrier.
A conventional technology for the MC-CDMA system is disclosed in U.S. Pat. No. 5,729,570, which method involves multiplying each user signal by an orthogonal code for spreading and transmitting the multiplied user signal on multi-carriers. This method guarantees a diversity effect in a frequency-selective fading environment, but generates an interference signal due to a loss of orthogonality among the user signals under frequency selective fading channels.
FIG. 1 is a schematic of a channel transmission symbol generating system in an MC-CDMA system according to prior art.
The channel transmission symbol generating circuit of the conventional MC-CDMA system comprises, as shown in FIG. 1, a code spreader 101, an interleaver 102, a serial-to-parallel converter 103, an inverse fast Fourier transform (IFFT) section 104, a parallel-to-serial converter 105, and a guard time inserter 106.
The code spreader 101 multiplies the data symbol of each user by an N-dimensional orthogonal code to spread the data symbol at a ratio of 1:N.
The interleaver 102 interleaves the spread signal from the code spreader 101 using an M by N block interleaver.
The serial-to-parallel converter 103 converts the serial chip stream into the parallel stream with size of MN chips. The IFFT section 104 receives the interleaved and serial-to-parallel converted signal and converts each sub-carrier signal to a time-domain signal to generate a multi-carrier signal.
The parallel-to-serial converter 105 converts the array of the signals from the IFFT section 104 to a serial arrangement.
Finally, the guard time inserter 106 adds a cyclic prefix to the parallel-to-serial converted signals to insert a guard time for transmission.
This MC-CDMA system enables a receiver to achieve a diversity effect in a frequency-selective fading environment by combining information on the individual carriers.
However, the non-uniform channel gain of each carrier causes a loss of orthogonality among users while combining chips at the receiver, thus generating an interference signal between the users. The interference component, if comparable to the thermal noise component, may have an critical effect on the system performance.